Recently, there is a growing demand for a power converter (inverter) that can drive a load operating with alternating current such as a motor as efficiently as possible with its overall size and cost reduced as much as possible. To meet such a demand, a field effect transistor made of silicon carbide has been developed as a switching element for a power converter.
Silicon carbide (SiC) is a high-hardness semiconductor material with a greater bandgap than silicon (Si), and has been used extensively in various kinds of semiconductor devices including power elements, hostile-environment elements, high temperature operating elements, and radio frequency elements. Among other things, the application of SiC to power elements such as semiconductor elements and rectifiers has attracted a lot of attention. This is because a power element that uses SiC can significantly reduce the power loss compared to a Si power element. In addition, by utilizing such properties, SiC power elements can form a smaller semiconductor device than Si power elements.
A metal-insulator-semiconductor field-effect transistor (MISFET) is a typical semiconductor element among various power elements that use SiC. In this description, a MISFET of SiC will sometimes be simply referred to herein as a “SiC-FET”. And a metal-oxide-semiconductor field-effect transistor (MOSFET) is one of those MISFETs.
FIG. 17 illustrates an example of a known three-phase power converter that uses SiC-MISFETs as switching elements. As shown in FIG. 17, the three-phase power converter 160 has three legs L1 to L3, which are provided for the three phases, respectively, and each of which has upper and lower arms that are connected together in series. Each of the upper and lower arms includes a switching element 1100 and an external freewheeling diode 1200 which is connected anti-parallel to the switching element 1100. The upper and lower arms ordinarily turn ON and OFF alternately, thereby changing the direction of current flowing through a load 1500. In FIG. 17, the current flowing from the switching element 1100 toward the load 1500 is identified by If. In this case, if the respective switching elements 1100 of the upper and lower arms turn ON simultaneously, then the power supply 2100 will get short-circuited and a huge amount of current will flow and cause a breakdown in the three-phase power converter 1600. Thus, to avoid such a situation, there is an interval in which the switching elements 1100 of both of the upper and lower arms are turned OFF (i.e., a so-called “dead time”) when the arms are changed from the upper arm into the lower one, or vice versa. A load on a power converter usually includes an inductive load. That is why even during that dead time, a voltage is also generated between the two terminals of the load in order to make the current that has just stopped flowing through the load start to flow again. As a result, inductive current Ir is generated. And to make that inductive current Ir flow, the freewheeling diode 1200 is connected anti-parallel to the switching element 1100.
As the freewheeling diode 1200, an external diode is generally used. Or if the switching element 1100 is a MISFET, then a pn junction inside of the semiconductor element that works as the MISFET may be used as a “freewheeling diode”. Such a pn junction functions as a kind of diode, and therefore is called a “body diode”.
It is believed that in a situation where the body diode is used as a freewheeling diode, if return current flows through the pn junction of SiC, then the degradation of crystallinity of a SiC-FET will proceed due to a bipolar operation performed by the body diode (see, for example, Patent Document No. 1 and Non-Patent Documents Nos. 1 and 2). If the degradation of crystallinity of a SiC-FET proceeds, the ON voltage of the body diode could rise. Also, if a body diode is used as a freewheeling diode, a reverse recovery current, which is generated by the reverse recovery of minority carriers of the pn junction, will flow due to the bipolar operation performed by the pn junction diode when the diode in ON state changes into OFF state. And that reverse recovery current causes not only recovery loss but also a decrease in switching rate as well.
Thus, in order to overcome such a problem involved by using a body diode as a freewheeling diode, it was proposed (in Patent Document No. 2, for example) that a return current is made to flow through a freewheeling diode element as an external electronic part by connecting the freewheeling diode element and an SiC-FET in anti-parallel with each other. As a freewheeling diode element for a SiC-FET, a SiC-SBD which would generate almost no reverse recovery current (or recovery loss) is suitably used. However, such a SiC-SBD is expensive and requires an increased number of parts, thus leading to an increase in circuit cost.
Thus, to overcome such a problem, a SiC-MISFET, in which return current is made to flow through the channel of a MISFET, was proposed (in Patent Document No. 3, for example).